PROJECT SUMMARY The primary objective of this research initiative is to evaluate how the metal-sequestering human host-defense protein calprotectin (CP) affects metal homeostasis and physiology of bacterial pathogens. Metal ions are essential nutrients for all organisms, and pathogens must acquire these nutrients from the host to replicate and cause infection. During this process, the human innate immune system works to limit the bioavailability of transition metals including manganese (Mn), iron (Fe), and zinc (Zn) by deploying CP and other metal- sequestering proteins at sites of infection. CP is accepted to withhold Mn(II) and Zn(II) from microbial pathogens, and we recently demonstrated that CP coordinates Fe in the reduced ferrous oxidation state. Bacterial systems for Fe(II) acquisition are increasingly appreciated as critical for pathogenesis in multiple infection states, including chronic, biofilm-mediated infections where oxygen becomes limiting. However, no other host-defense proteins that limit Fe(II) have been identified; thus investigating CP as an Fe(II)- sequestering host-defense protein is important for understanding host-pathogen interactions in chronic infection states. Pseudomonas aeruginosa (Pa) and Staphylococcus aureus (Sa) are two human pathogens that cause chronic polymicrobial infections in diverse patient populations, including lung infections in individuals with cystic fibrosis (CF). This hereditary disease predisposes individuals to life-long pulmonary infections, marked by debilitating exacerbations that reduce lung function. Notably, the CF lung becomes increasingly hypoxic as disease progresses, and multiple lines of evidence indicate that Fe(II) becomes the predominant form of bioavailable Fe. Progression of CF lung disease is also correlated with a shift in microbial etiology, with Sa being the predominant microorganism in younger patients, and subsequent Pa colonization associated with lung function decline. The underlying biology that causes this population shift remains poorly understood; however, recent studies suggest that both Fe and CP contribute to this process. We hypothesize that CP limits Fe(II) availability in hypoxic environments, as found in the CF lung, and that this activity eventually allows Pa to outcompete Sa in polymicrobial environments. In Aim 1, we will evaluate Fe(II) sequestration by CP, and map the distribution of metal ions in Pa and Sa cultures treated with CP. In Aim 2, we will test the hypothesis that CP limits Fe(II) to Pa and Sa, and thereby impacts the individual physiologies and co-culture dynamics of these two pathogens. These investigations will enable future studies that address how CP and Fe drive the progression of CF lung infections, and may guide the design and development of novel diagnostic, preventative, and therapeutic approaches to treat bacterial infections.